1. Field of the Invention
The present invention concerns an improved prism assembly for use in optical devices, such as in 3-chip DLP-projectors.
Digital light processing (DLP™) technology is often used in digital projectors. DLP is a trademark of Texas Instruments in the United States of America. These projectors use a digital multi-mirror device (DMD) to modulate the light.
Each pixel in the chip can direct the light that hits it in either the ‘on state’ or the ‘off state’. The on state light reaches the screen and gives a white pixel, while the off state light is absorbed inside the projector; this results in a dark pixel.
For best light-output and saturated colors, it is best to use three DMD's, one for the red, green and blue light.
2. Discussion of the Related Art
Currently, two architectures are used: a 5-element prism assembly (also called “Philips-prism”) and a 6-element prism assembly, which both have different disadvantages. As shown in FIGS. 1 and 2, with the classical 5-element solution (Philips prism), the three DMD's are combined using five individual prisms.
The TIR (total internal reflection) assembly 1 consists of two prisms 2 and 3. There is a small air gap between these two prisms (in the order of 10 μm). The function of the TIR-prisms is to direct the incoming light to each of the respective the DMD's 4 to 6, while allowing the on state light to exit.
The color prism assembly 7 consists of three prisms 8 to 10. Their function is to split the incoming light A into three spectral bands B, C and D, one for each DMD 4 to 6. This is achieved with two dichroic coatings. After reflection and modulation of the light at the DMD, the color prisms 8 to 10 recombine the light. There are also air gaps between the three prisms 8 to 10 of the assembly 7.
The TIR-prism assembly 1 and the color prism assembly 7 are mounted so that an air gap 11 of about one millimeter exists between the two.
A disadvantage of the classical five-element solution is the “dichroic shift light” in the color prisms 8 to 10.
This is caused by the dichroic coatings, which are used for separating and combining the three colors. The root cause is the angle dependence of these dichroic coatings.
The incoming bundle of light (separation) has a certain angle of incidence on the respective DMD, determined by the tilting angle of the DMD, whereas the exiting bundle (recombination) has a right angle on the DMD. Both bundles have thus different angles of incidence on the dichroic coatings and the characteristics of the dichroic coatings are different for both bundles.
The problem arises when a coating reflects light during the separation process, but then transmits it during the recombination. It also arises in the complementary situation, when a coating transmits light during the separation process, but then reflects it during the recombination.
This dichroic shift light is undesirable, because it heats up the prism assembly and it can end up on the screen as stray light, which lowers contrast.
The classical 6-element prism assembly, which is shown in FIGS. 3 and 4, was developed in order to reduce the dichroic shift light. There is an extra element 12 between the TIR prism assembly 1 and the color prism assembly 7. This sixth element 12 allows a big reduction of the angle of the blue dichroic coating, at the expense of a small increase of the angle of the red dichroic coating. The smaller these angles, the better the coatings perform with respect to dichroic shift light.
A second advantage of the 6-element prism assembly is increased transmission, because of the improved performance of the coatings.
An important disadvantage of the 6-element prism assembly is the increased cost.
Another disadvantage is the increased required optical path length, measured along the optical axis. This makes it more difficult and costly to design and manufacture a suitable projection lens.